Conventional lenses are typically formed of a number of varying thickness pieces of glass or plastic to make compound lenses of a sufficient f/#, and to provide better imaging quality. In particular, fast low f/# lenses, e.g., f/1 or f/2, can become very bulky and heavy. The size and weight of such lenses presents deployment problems, such as for night-vision equipment, or for optical systems aboard UAVs (unmanned aerial vehicles) for persistent surveillance.
The recent development of Transformation Optics (TO) provides a new way of looking at the independent control of the electrical and magnetic components of electromagnetic fields [see Pendry, J. B., Schurig, D. & Smith, D. R., (2006) ‘Controlling electromagnetic fields’, Science, 312, pp. 1780-1782]. TO is enabled in practice through the use of metamaterials. As disclosed in Pendry et al. “ . . . metamaterials owe their properties to their sub-wavelength material-structure rather than to their chemical composition, and can be designed to have properties impossible to find in nature.”
At the sub-wavelength level, light breaks up into its component electrical and magnetic fields, and the concept of a ray of light is meaningless. In this case, TO replaces Snell's Law of Refraction. TO is the valid mathematics at the sub-wavelength scale.
Ishii et al. describe metal nano-slit lenses using a polarization-selective design [Ishii, S. et al., (2011) ‘Metal nano-slit lenses with polarization selective design’, Optics Letters, 36, pp. 451-453]. The nano-slit lenses described in Ishii et al. had a design of nano-slits formed in a gold thin film. Both concave and convex lenses are described in Ishii et al., but such lenses have a serious drawback for general use—they only work for a certain plane polarization of light to be imaged.
Lenses with negative refractive index metamaterials are known. With such negative refractive index metamaterials the goal is perfect lenses with unlimited resolution. Shalaev et al. [Shalaev, V. M., et al., (2005) ‘Negative index of refraction in optical metamaterials’, Optics Letters, 30, pp. 3356-3358] discuss the use of close-spaced gold nano-rod pairs in air on top of a dielectric substrate to create a metamaterial for the purpose of creating negative refractive index materials.
Kildishev and Shalaev disclose the design of cylindrical-lenses for super-resolution and optical-cloaking [Kildishev, A. V. & Shalaev, V. M., (2011) ‘Transformation optics and metamaterials’, Physics-Uspekhi, 54, pp. 53-63]. Such cylindrical lenses, however, are not for use in everyday imaging systems. These cylindrical lenses require the object to be imaged to be placed inside the cylindrical lens. Cylindrical metamaterial lenses are of the wrong geometry and scale for most uses. Moreover, the cylindrical lenses of Kildishev and Shalaev were of a negative refractive index design in a metal-dielectric system.
West et al. [West, P. R., et al., (2010) ‘Searching for better plasmonic materials’, Laser Photonics Reviews, 4, pp. 795-808] have identified that transparent conductive oxides may be used to construct lower loss negative index materials than can be created using metals such as gold, silver or copper etc. The oxides and nitrides identified include indium tin oxide (ITO), Al:ZnO and Ga:ZnO. TiN and ZrN are also found to be useful nitrides for metal replacement. West et al. have speculated that the benefit of oxide and nitride replacements for metals such as gold in metamaterial construction is likely to be found in the visible and near IR spectral regions for negative index material construction. The focus of West et al. is to make negative refractive index materials for the creation of ‘perfect lenses’ etc.